Table 191

Selected Actin-Binding Proteins Protein MW Domain Organization* Location

CH-Domain Superfamily Fimbrin 68,QQQ

"CO

Microvilli, stereocilia, adhesion plaques, yeast actin cables a-Actinin

1Q2,QQQ

Filopodia, lamellipodia, stress fibers, adhesion plaques

Spectrin a: 28Q,QQQ Q^3

Cortical networks

Dystrophin

427,QQQ

eQ Muscle cortical networks

Filamin

Others Fascin

28Q,QQQ

55,QQQ

Filopodia, pseudopodia, stress fibers

Filopodia, lamellipodia, stress fibers, microvilli, acrosomal process

Villin

92,QQQ

Microvilli in intestinal and kidney brush border

*Blue = actin-binding domains; red = calmodulin-like Ca +-binding domains; purple = a-helical repeats; green = p-sheet repeats; orange = other domains.

Actin filament

Actin filament

a between orthogonally arranged and loosely packed filaments (see Figure 19-5b). The large networks thus formed fill the cytoplasm and give it a gel-like character. Because these proteins also bind membrane proteins, the networks are generally found in the cortical region adjacent to the plasma membrane. In proteins that form networks of filaments, repeats of different protein motifs determine the length of the arms and thus the spacing and orientation between filaments.

Although CH-domain proteins form the majority of actin cross-linking proteins, other proteins that bind to different sites on actin play equally important roles in organizing actin filaments. One such protein, fascin, is found in many actin bundles including stress fibers, cell-surface mi-crovilli, and the sensory bristles that cover the body of the fruit fly Drosophila. The important structural role of fascin is illustrated by the effects of mutations in the singed gene, which encodes fascin in Drosophila. In singed mutants, sensory bristles are bent and deformed, evidence that fascin is responsible for maintaining the rigidity of actin bundles in the core of each bristle.

▲ FIGURE 19-5 Actin cross-linking proteins bridging pairs of actin filaments. (a) When cross-linked by fimbrin (red), a short protein, actin filaments pack side by side to form a bundle. (b) Long cross-linking proteins such as filamin are flexible and can thus cross-link actin filaments into a network. [Part (a) courtesy of D. Hanein. Part (b) courtesy of J. Hartwig.]

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